An integrated circuit (IC) for battery monitoring requires a reference voltage. A reference voltage generating circuit using a Brokaw cell is a known circuit that uses the band gap voltage of a coupled transistor to generate a reference voltage. In this circuit, however, the temperature dependence of the reference voltage is shaped like a convex parabola, which is undesirable. Therefore, a band gap voltage reference circuit that generates a flat reference voltage over a wide temperature range by adopting a temperature compensating circuit that corrects the temperature dependence in low-temperature region and high-temperature region has been previously developed.
That is, a Brokaw cell in this reference voltage generating circuit is equipped with two bipolar transistors Q1 and Q2 having emitter areas with an area ratio of A:1 (where A is an integer value) and an operational amplifier that differentially amplifies the voltage between the collectors of the transistors Q1 and Q2. The Brokaw cell is also equipped with a feedback loop that feeds back the amplified output of the operational amplifier to the bases of the transistors Q1 and Q2. The Brokaw cell also includes two resistors R1 and R2 connected in series between the emitter of the transistor Q1 and the ground potential. In this Brokaw cell, the voltage VBE between the base and emitter of the transistor Q2 has a negative temperature coefficient that decreases with respect to the temperature rise. Also, a voltage having a positive temperature coefficient that increases with respect to temperature rise is generated across the resistor R2. The two voltages having the positive and negative temperature coefficients act on the voltage at the connection point of resistors R1 and R2.
A temperature compensating circuit may include a first differential amplifier that compensates for the temperature dependence in the low-temperature region and a second differential amplifier that compensates for the temperature dependence in the high-temperature region. Current from constant current sources are supplied to these differential amplifiers. The output currents are supplied to a mirror circuit, whose output corrects the output voltage of the Brokaw cell.
In the prior art, however, there are usually variations in the characteristics of the elements, such as the transistor pair Q1 and Q2 of the Brokaw cell, and the resistors R1 and R2. The temperature dependence of the output reference voltage, which should be flat with respect to temperature variation, tends to tilt with a positive or negative slope due to the variations in the collector currents of the transistors Q1 and Q2. The value of the supplied correction current also varies due to the variations in the elements, making a reference voltage generating circuit unable to output a constant reference voltage with high accuracy.
More specifically, due to the variations in the elements during fabrication, the contribution of the voltage having the positive temperature coefficient present at the connection point of the resistors R1 and R2 or the contribution of the base-emitter voltage of transistor pair Q1 and Q2 may become too strong or too weak relative to the negative temperature coefficient, and the overall temperature characteristic (dependence) of the reference voltage will therefore tend to have a positive or negative slope. The reference voltage may also vary due to unintended variations in the current sources that supply, notionally, constant currents to the first and second differential amplifiers of the temperature compensating circuit.